This invention relates to scanning devices whereby a target within a conical scanning field may be scanned so as to obtain a three dimensional scanned image. The target is spirally scanned. The invention also provides for volumetric scanning of a target within the conical scanning field. The invention is particularly directed towards ultrasonic scanners, but optical scanners are also contemplated.
Typically, the present invention is directed to an ultrasonic scanning apparatus wherein an ultrasonic transducer is mounted for continuous rotation about a pivot point, with motive force provided by electromagnetic means under digital electronic control. Preferably, a scanning apparatus in keeping with the present invention employs a transducer assembly which is disposed within a hand-held probe.
Briefly, such an ultrasonic scanner has a housing, and an ultrasonic transducer disposed within the housing and mounted therein for two-way tilt motion about a pivot point by means of a gimbal or similar mechanism. A permanent magnet is affixed to the rear of the transducer, and electromagnetic means are provided to effect movement of the transducer/magnet assembly. The electromagnetic means comprises at least two electromagnet coils which jointly form a hemispheric electromagnet coil assembly. The electromagnetic coils are driven by polyphase sinusoidal electric currents, causing rotation of the transducer about an axis. Periodic amplitude variation of the polyphase electric currents will effect periodic change of the angle between the axis and a fixed central axis, thereby causing the ultrasonic beam to sweep out a conical volumexe2x80x94and thereby resulting in volumetric scanning.
Electrical means are provided for energizing the transducer and receiving signals therefrom which are indicative of reflected scanning energy which is reflected back from a target in the conical scanning field. Digitizing means are provided to convert the received signals to digital form for storage and/or display.
Sequential control means are provided to energize the electromagnet coils with precise phasing, and further more to synchronize transducer motion, periodic energization of the transducer, and digitization of signals received from the transducer. Of course, suitable storage and/or display means are provided to store and/or to present the signals received by the transducer to an operator, in any one of several time-varying graphical representations.
In an extension of the present invention, the transducer may be replaced by a mirror, with an external light source being arranged to direct light onto the mirror from whence it will be re-directed and transmitted linearly outwardly therefrom.
The following is a list of issued United States patents and Published Canadian Patent Applications which are referred to hereafter:
Ultrasonic imaging, also called echography or B-mode (xe2x80x9cbrightness modexe2x80x9d) ultrasound, involves an ultrasonic transducer which repeatedly emits pulses of high-frequency sound and receives the resulting echo signals. A focused beam of sound is normally used, and various means are employed to sweep this beam repeatedly through a range of directions. Electronic processing of the received echo signals, synchronized to the movement of the beam, results in formation of a video image (normally cross-sectional) of structures (such as human tissues) in the beam""s path.
For purposes of clarity, the following discussion refers specifically to the medical diagnostic imaging situation in which the target of the ultrasonic imaging system is tissue in a living human patient. It should be realized that ultrasonic scanning is also used in other applications including non-human (veterinary or in vitro tissue sample) biological imaging and also non-biological imaging applications (non-destructive materials testing), and that the present invention applies without limitation to all of such ultrasonic imaging applications.
In ultrasonic scanners for medical diagnostic imaging, the transducer and beam-sweeping components are normally assembled in the form of a hand-held probe connected to the rest of the imaging system by means of a cable. In the following discussion, the term xe2x80x9cprobexe2x80x9d will be used generically to refer to the transducer and beam-scanning assembly, though it should be realized that not all ultrasonic scanning systems will have or need a physically distinct probe component.
Historically, B-mode ultrasound imaging has been a two-dimensional process. The sweep motion of the sound beam emitted and received by the transducer is confined to a single plane intersecting the target to be imaged, and a cross-sectional image results. U.S. Pat. Nos. 5,159,931, 5,454,371, 5,562,095, 5,842,473, and 5,964,707 describe methods by which three-dimensional images may be assembled from multiple planar echographic images. However, it is also possible to reconstruct three-dimensional images directly, by causing the transducer to move with two degrees of freedom, with the result that the sound beam sweeps throughout the target volume in some predetermined (programmed) path. The present invention relates to this latter approach, and is referred to as volumetric scanning.
Many techniques for sweeping the sound beam are known; U.S. Pat. No. 4,092,867 presents a good summary. Today, two main approaches are in common use. In the mechanical sector-scan approach, a transducer is mechanically oscillated about a pivot axis, causing the sound beam to sweep through a sector. In the electronic transducer array approach, a fixed array of multiple transducer elements is used, and the sound beam is formed, focused, and swept entirely electronically.
Attempts to use the electronic array approach for volumetric scanning, using a two-dimensional array of transducer elements, have not yet proved practical, and in any event the cost and complexity of the electronics to process the hundreds of individual element signals which would be required are formidable. In contrast, the simple mechanical sector-scan approach can in principle be adapted for volumetric scanning by permitting the transducer to oscillate about a pivot point rather than a pivot axis, e.g. using a gimbal or similar mechanism.
Any number of sweep paths may be envisaged, and if the electronic array approach is used almost any path might reasonably be used to effectively scan a target volume at high speed. With mechanical scanning, however, the only practical sweep path is a circular spiral. Non-spiral paths require rapid changes of the acceleration vector, which in practice will greatly limit the operating speed. Rapid scanning is desirable because it permits dynamic imaging in the presence of motion, e.g. of the living fetus in utero, of the tissues in the eye, or in the course of interventional medical procedures such as catheterization or laparoscopic surgery. The present invention relates to mechanical, spiral scanning of a single transducer mounted for motion with two degrees of freedom in e.g. a gimbal.
Because the sound frequencies used in ultrasonic imaging are effectively blocked by air, it is necessary to acoustically couple the transducer to the target under investigation via one or more acoustically conductive media. Coupling media must be chosen carefully, to ensure that the effects of sound reflection and refraction occurring at media interfaces does not unduly compromise the imaging capabilities of the system. In practice, the problem of coupling is addressed in one of three ways:
1. Liquid bath. Transducer and target are immersed in a liquid coupling medium such as water or sterile saline solution. This technique is rarely used in clinical scanning applications.
2. Sealed transducer chamber. The transducer is sealed within a liquid-filled chamber within the probe, with a solid xe2x80x9cacoustic windowxe2x80x9d through which the imaging beam can pass.
3. Acoustic coupling gel. Several water-based gel media are now on the market, which provide a convenient means to couple a probe""s acoustic window to the surface of target tissue.
In any mechanical scanning approach, acoustic coupling to target tissue may be achieved either by method 1 above, or more commonly, a combination of methods 2 and 3. The present invention also discloses some specialized variations of methods 2 and 3.
So-called annular array transducers, which consist of a small number of concentric ring-shaped transduction elements, may be used in place of a single-element transducers in a mechanical sector scanning apparatus. These devices permit electronic control of sound beam focusing, but not direction. Because the number of transduction elements is small (e.g. three to eight), the cost premium associated with annular array transducers is slight compared with that of a full electronic array beam sweeping system. In this application, the term xe2x80x9ctransducerxe2x80x9d refers equally to either a single element transducer or an annular array transducer.
Motive force in mechanical sector scanning devices frequently involves use of an ordinary rotating electric motor plus some form of mechanical linkage to transform rotational motion to back-and-forth oscillatory motion of the transducer. U.S. Pat. No. 5,357,963 discloses a variation of the standard linkage in which the force of attraction between two permanent magnets is used to effect transfer of motive force through the wall of a sealed fluid-filled transducer chamber. U.S. Pat. No. 4,732,156 discloses a similar concept in which the sealed transducer chamber and magnetic linkage are disposed at the end of a flexible mechanical coupling shaft to facilitate insertion of the whole assembly into a body cavity, and the sound beam from a fixed transducer is caused to scan by means of a rotating mirror.
Purely mechanical means of oscillation, with or without use of magnetic elements on some of the moving members to transfer force across a gap, exhibit a number of disadvantages. The mechanical drive assembly adds substantially to the size and weight of the imaging probe, and to achieve precisely controlled motion it is necessary to augment the mechanical system with a positional sensor and to employ closed-loop feedback control methods in the control system. Addition of the positional sensor adds yet further to the size, weight, and complexity of the probe, while the need for closed loop feedback control adds to the complexity of the control system.
U.S. Pat. No. 4,092,867 discloses two mechanisms in which motive force is applied by means of magnetic forces established between fixed electromagnets, energized with time-varying electric current, and a single permanent magnet affixed to a rotating transducer assembly. U.S. Pat. Nos. 5,647,367 and 5,701,901 describe broadly similar mechanisms suitable for micro-fabrication. In all of these cases, the magnetic oscillator mechanism follows the general principle of a galvanometer, i.e., a magnetic field is established in a fixed stator member, with the magnetization vector substantially at right angles to that of a permanent magnet affixed to a pivoted rocker member. The magnet in the rocker is thus subjected to magnetic torque forces which cause it, and the rocker, to tilt or rotate so as to reduce the angle between the two magnetic vectors. Periodically reversing the direction of the stator field, by passing an alternating current through the one or more electromagnet coils in the stator assembly, thus results in a periodically reversing torque applied to the rocker, and in consequence causes the rocker to oscillate.
In practice, such galvanometer-like mechanisms cannot transfer appreciable amounts of mechanical energy, because of the inverse square relationship between magnetic force and distance. As the rocker magnet tilts away from the center position, it moves further from the stator magnet poles, dramatically reducing the torque efficiency of the system and making it harder to apply the reverse torque required to tilt the rocker back in the opposite direction. Such a system is rather like a heavy weight balanced atop a rod held in the hand; provided the rod does not tip very far from the vertical, small movements of the hand suffice to keep it stably aloft, but the range of controllable motion is very small. U.S. Pat. No. 4,092,867 explicitly acknowledges the need for an active feedback control system to maintain stable motion, while U.S. Pat. Nos. 5,647,367 and 5,701,901 address the problem in a limited way by proposing a specially fabricated rocker of near-zero mass driven by a stator electromagnet of enormous relative size. In both cases the result is a probe mechanism substantially larger than the size of the ultrasonic transducer itself.
U.S. Pat. No. xe2x80x9cunknownxe2x80x9d (to issue from Ser. No. 09/409,095) discloses an invention which addresses the torque efficiency problems by using a system of many digitally switched stator electromagnet coils, which permit periodic alteration of the magnetic field vector while maintaining a small and little-varying gap between the currently active stator poles and a permanent magnet in the rocker; and furthermore solves the problem of stability at higher operating speeds by providing additional permanent magnets affixed to the stator near the extremes of the rocker""s oscillatory motion, oriented so as to repel the permanent magnet in the rocker, causing it to spring back in the opposite direction.
In all of these cases, the most difficult problem to overcome is the need for periodic reversal of the force applied to the transducer, in order to effect back-and-forth motion. A spiral-scan approach sidesteps this problem, however. Circular motion can be effected by means of a continuously-rotating, centrally-directed (centripetal) force vector; gradually increasing and decreasing the magnitude of this rotating vector results in spiral motion. In the present invention, this is accomplished using electromagnetic means.
Reconstruction of an ultrasound image requires means to correctly locate each image point within the visual display. In a modern digital imaging system, the display is usually formed using a raster-scan system comprising a regular, Cartesian-coordinate grid of discrete points or pixels, whereas the acquisition of image points occurs, due to sector scanning, on some form of Polar-coordinate grid. Image reconstruction is hence a coordinate transformation process which is amenable to digital implementation using a fixed coordinate lookup table. With spiral volumetric scanning as in the present invention, image data points are most naturally defined on a spherical-coordinate grid, and there is the added problem that while image data are acquired throughout a three-dimensional (conical) volume, they must somehow be displayed on a two-dimensional surface, e.g. that of a cathode-ray tube or flat-panel display.
Image reconstruction cannot be performed with any fidelity unless the precise geometric location of each acquired image point is accurately known. In a practical continuous-sweep imaging system, this is essentially a matter of proper synchronization of the mechanical and electronic apparatus. Historically, various means of instrumenting the physical motion of a transducer have been used to provide a feedback signal to be used in a closed-loop synchronization system. With electronic array scanning techniques, there is no physical motion and hence the problem is greatly simplified; no feedback is required. U.S. Patent No. xe2x80x9cunknownxe2x80x9d (to issue from Ser. No. 09/409,095) discloses a mechanical sector-scanning apparatus in which motive force is provided by a system of digitally switched electromagnets acting upon a permanent magnet affixed to the rear of a pivoted transducer. Such a system can operate reliably without feedback. The present invention takes a somewhat similar approach, modified for transducer motion with two degrees of freedom.
U.S. Pat. Nos. 5,454,371 and 5,842,473 disclose digital image reconstruction and display techniques whereby three-dimensional volumetric image data may be presented on a two-dimensional display surface, through use of three-dimensional linear coordinate transformations. However, the manner in which image reconstruction and display may be accomplished is outside the scope of the present invention.
A typical practical embodiment of the present invention provides a hand-held probe which carries therewithin an ultrasonic transducer, and is such as to accomplish volumetric ultrasound scanning of a target within a conical scanning field. In such a probe, in keeping with the present invention, a permanent magnet is affixed to the rear of a carriage containing the ultrasonic transducer. The carriage is mounted for tilt motion with at least two degrees of freedom about a central pivot point by means of a gimbal or similar mechanism. The carriage and pivot mechanism are disposed within a stator assembly containing at least two fixed, hemispheric electromagnet coils. The whole is contained within a probe housing having an acoustically transparent window at the end closest to the transducer, through which the emitted and received sound pluses travel.
In operation, the electromagnet coils are energized with appropriately phased alternating electric currents, in order to cause the carriage assembly to rotate about the central pivot point. The speed of rotation is set by the frequency of the drive current; the amount by which the rotation axis tilts outward from the central axis is set by the drive current amplitude. Periodic variation of the drive current amplitude causes periodic variation of the tilt angle, in effect causing in-and-out spiral motion of the transducer.
The electromagnet coils are energized by means of a digital electronic switching system, resulting in highly repeatable motion of the transducer. In applications where continuous feedback control based on direct sensing of the transducer""s position is not practical (e.g., when practical size constraints of an ultrasonic imaging probe do not permit inclusion of sensor components), the motion may be calibrated once using external feedback devices, and will remain stable long enough to permit accurate images to be acquired. Thus a three-dimensional ultrasound tomographic imaging system may be implemented with a minimum of components and a very small and simple probe having but one moving part.
This basic design admits a number of useful variations, including without limitation the following:
1. Instead of disposing the transducer within a moving carriage, the carriage may instead contain an acoustic reflector to reflect and sweep the sound beam emitted by a fixed ultrasonic transducer.
2. The ultrasonic transducer may be replaced by another energy source and/or detector. In particular, an optical source such as a semiconductor diode laser or superluminescent diode, or one end of a flexible optical fiber coupled to a fixed energy source and/or detector at the far end of the fiber.
3. Although in ultrasonic imaging it is most common to utilize a single transducer for emission and detection of sound, in optical imaging either the source or detector device may be fixed. For example, with respect to variation #2 above, the light beam from a semiconductor diode type light source could be swept across the target area while reflections are received by a fixed, wide-field detector such as a photomultiplier tube.
The present invention provides a scanning device for scanning a target within a conical scanning field, using a moveable body from which scanning energy may be transmitted linearly outwardly toward a target, and from which signals which are indicative of reflected scanning energy which is reflected from a target in the conical scanning field, may be derived for further storage or display. The scanning device comprises:
A housing for the scanning device, which housing has a longitudinal axis.
A moveable body having a generally planar front surface from which scanning energy is transmitted linearly outwardly, and towards which reflected scanning energy is directed.
A permanent magnet which is physically associated with the moveable body so as to be moveable therewith.
The moveable body and permanent magnet are mounted within the housing in such a manner that they are jointly moveable with a tilt motion about a centre of rotation, with at least two degrees of freedom of movement about two axis of rotation which are perpendicularly disposed one to the other. The centre of rotation is located on the longitudinal axis of the housing; and the two perpendicularly disposed axis of rotation intersect at the centre of rotation, and are each perpendicular to the longitudinal axis of the housing.
At least two electromagnets having wound electromagnet coils form a hemispheric electromagnet coil assembly.
There is an electric drive means for energizing the wound electromagnet coils in cyclic fashion, by applying an alternating current signal to each individual wound electromagnet coil, where the signal applied to each wound electromagnet coil has a differing phase than the signal applied to any other wound electromagnet coil.
The alternating current signal which is applied to each of the wound electromagnet coils is out of phase with the alternating current signal applied to any other wound electromagnetic coil; and the phase relationship among the respective alternating current signals is 180xc2x0/n, where n is the number of wound electromagnet coils in the hemispheric electromagnet coil assembly.
Thus, when scanning energy is transmitted linearly away from the moveable body, and the hemispheric electromagnet coil assembly is energized by the electric drive means, a conical scanning field is swept.
The present invention further provides for either or both of the magnitude or frequency of the alternating current signal applied to each of the wound electromagnet coils to be periodically modulated so as to cause the conical path swept by the transmitted scanning energy to alternately narrow and widen. Thus, a target in the conical scanning field of the scanning device is volumetrically scanned.
In many practical embodiments of the present invention, the moveable body is an ultrasonic transducer which is mounted at a first end of a moveable carriage, and the permanent magnet is mounted at an opposed second end of the moveable carriage.
In other practical embodiments of the present invention, the moveable body is a mirror, and a source of scanning energy is directed towards the mirror in a manner so as to be re-transmitted linearly away therefrom.
In any embodiment of the present invention, the moveable body and the permanent magnet may be gimbal mounted within the housing.
In such instances, it is possible that the gimbal may have knife-edge bearings.
In other embodiments of the present invention, the moveable body and the permanent magnet may be mounted on an elastomeric membrane which is secured within the housing.
In a further embodiment of the present invention, an annular permanent magnet is mounted in the housing outwardly of the hemispheric electromagnet coil assembly. The annular permanent magnet has a polarity opposite to that of the permanent magnet mounted together with the moveable body, so as to repel the permanent magnet away therefrom and towards the longitudinal axis of the housing.
Another embodiment of the present invention contemplates a further solenoid electromagnet located on the longitudinal axis of the housing at the side of the hemispheric electromagnet coil assembly which is remote from the permanent magnet. A source of direct current electricity is connected to the solenoid electric magnet. The solenoid electromagnet may be momentarily energized by the source of direct current electricity so as to either attract or repel the permanent magnet.
The scanning device of the present invention may be employed in an ultrasonic scanner together with signal handling means for processing signals which are indicative of the scanning energy which is relucted from a target in the conical scanning field, so as to derive an image therefrom. Display means may be provided for displaying that image.
When the scanning device of the present invention includes an ultrasonic transducer, there may be an acoustic window which encloses the first end of the housing, and enclosure means enclosing an opposed second end of the housing, so that all of the ultrasonic transducer, the permanent magnet, and the hemispheric electromagnet coil assembly, are contained within the housing together with an ultrasonic sound conducting fluid.
In another embodiment of ultrasonic scanning device of the present invention, the ultrasonic transducer and the permanent magnet are enclosed within a capsule having an acoustic window at a first end thereof which is remote from the permanent magnet, together with an ultrasonic conducting fluid.
In still another embodiment of ultrasonic scanning device in keeping with the present invention, there is an acoustic window located at the end of the housing proximate the ultrasonic transducer, and a pliant and sealed acoustically transparent container of ultrasonic conducting fluid is mounted between the ultrasonic transducer and the acoustic window in acoustically conductive relation therewith.
A further embodiment of ultrasonic scanning device of the present invention contemplates a pliant and sealed acoustically transparent container of ultrasonic conducting fluid which is mounted in acoustically conductive relation with the ultrasonic transducer.
Still further, an ultrasonic scanning device in keeping with the present invention may comprise an acoustic window which is located at the end of the housing approximately ultrasonic transducer, and a pliant and highly elastic self-contained ball which may be an acoustically transparent elastomer, or gelatin, is mounted between the ultrasonic transducer and the acoustic window in acoustically conductive relation therewith.
Yet another embodiment of an ultrasonic scanning device of the present invention comprises a pliant and highly elastic self-contained ball which may be acoustically transparent elastomer or gelatin, and which is mounted in acoustically conductive relation with the ultrasonic transducer.